Processes for enrichment and/or separation of propane and propene (propylene) in a mixed C.sub.3 feed often rely on expensive distillation. The separation of propylene and propane in mixed C.sub.3 streams (or analogous close-boiling alkene/alkane mixtures containing butenes, amylenes, etc.) presents a difficult problem for refiners. These light hydrocarbons have boiling points separated by only a few degrees, which complicates their isolation into pure components through distillation. Other methods of separation, such as selective sorption using molecular sieves, are economically unattractive. Using current refinery technology, propene may be easily upgraded to gasoline components by alkylation, oligomerization, etherification, etc. However, in addition to propylene, many C.sub.3 streams also contain large amounts of propane, which is more difficult to convert to fuel range hydrocarbons. Use of mixed C.sub.3 feeds in propylene upgrading processes is hindered by the presence of the paraffin, which increases pumping requirements and limits the use of recycle for increasing olefin conversion. Economic techniques for separation of propylene and propane could increase the attractiveness of refinery processes which utilize the C.sub.3 olefin as well as provide a source of highly-concentrated olefinic and paraffinic streams.
Recent work in the field of olefin upgrading has resulted in catalytic processes for converting lower olefins to heavier hydrocarbons. Particular interest is shown in a technique wherein gasoline and/or distillate range hydrocarbons can be synthesized over ZSM-5 type medium pore zeolite catalysts at elevated temperature and pressure to provide a product having substantially linear molecular conformations due to the ellipsoidal shape selectivity of certain medium pore catalysts.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. In U.S. Pat. No. 4,227,992 Garwood and Lee disclose the operating conditions for the Mobil Olefin to Gasoline/Distillate (MOGD) process for selective conversion of C.sub.3.sup.+ olefins to mainly aliphatic hydrocarbons. In a related manner, U.S. Pat. Nos. 4,150,062 and 4,211,640 (Garwood et al) disclose a process for converting olefins to gasoline components.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, such as ZSM-5 type catalyst, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C.sub.10.sup.+ aliphatic product. Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions of the prior art do not convert a major fraction of ethylene. A typical reactive feedstock consists essentially of C.sub.3 -C.sub.6 mono-olefins, with varying amounts of nonreactive paraffins and the like being acceptable components for ordinary commercial purposes. These prior art oligomerization methods require relatively high temperature to provide adequate conversion, resulting in undesirable cracking reactions and a broad spectrum of carbon numbers in the products.
While low temperature oligomerization is known, prior catalysts have not shown sufficient activity below about 200.degree. C. to be practical in industrial applications. The advantages of low severity oligomerization with medium pore zeolites have been described by Avidan et al in U.S. Pat. Nos. 4,746,762 and 4,873,385. It is generally understood that low temperature oligomerization can be selective to produce incremental oligomers which have molecular weights as multiples of the monomers, such as isomeric propene oligomers consisting essentially of C6, C9, C12, etc. These reactions are selective without significant cracking of the desired product; however, the relative inactivity of prior art catalysts has prevented development of low temperture processes.
It is an object of this invention to provide an improved process for selective catalytic oligomerization of an olefinic feedstock which comprises contacting said feedstock under low severity catalytic conversion conditions with a novel acid solid catalyst having ultra-large pores and exceptionally high oligomerization activity at low temperature. The resulting oligomers are highly branched, incremental oligomers, which can be further converted under disproportionation or cracking conditions to yield lower olefins rich in tertiary compounds.